1. Field of the Invention
The present invention relates to an over-current protection device, and more particularly, to an over-current protection device that becomes opened during burning or failure.
2. Description of Related Art
As portable electronics such as mobile phone, laptop computer, portable video camera, personal digital assistant etc. are widely used, the importance of the over-current protection device, used to prevent electronic devices from the occurrence of over-current or over-temperature, is increased.
FIG. 1 is a schematic diagram of an over-current protection device 10 according to the prior art. The over-current protection device 10 includes a current-sensing element 11, an upper metallic conductive sheet 16 and a bottom metallic conductive sheet 15. The current-sensing element 11 includes an upper electrode foil 13, a bottom electrode foil 12 and a conductive material 14 with a positive temperature coefficient (PTC). The upper metallic conductive sheet 16 and the bottom metallic conductive sheet 15 connect the surface of the upper electrode foil 13 and the bottom electrode foil 12, respectively, which can further connect a conductive wire in series to an electronic circuit or connect the positive and negative electrode of a secondary batter. The PTC conductive material 14 is made of a polymer and conductive filler.
Since the resistance of the PTC conductive material 14 is very sensitive to temperature variation, the resistance is kept at an extremely low level during normal operation and the electric circuit operates normally. However, if an over-current or over-temperature event occurs, the resistance will be increased instantaneously to a very high level such as beyond 104 ohm to reversely eliminate the over-current and the achieve the protection of the electronic device.
However, when the conventional over-current protection device 10 is burned due to inappropriate use, the PTC layer 14 becomes carbonized and forms a highly conductive layer. Consequently, the upper metallic conductive sheet 16 and the lower metallic conductive sheet 15 connected to the positive and negative electrodes of the battery will be shorted, which is so called unsafe failure. As a result, the conventional over-current protection device 10 can not only achieve the circuit protection, but also endanger the use of the circuit system.
As the size of the secondary battery shrinks, the requirement for power efficiency and safety is increased. If the conventional over-current protection device 10 is assembled in an electronic device, there will be safety concerns for using the electronic device. Thus, it is necessary to provide an effective solution to this problem.